1. Field of the Invention
The present invention is directed to an improved process for the preparation of aqueous dispersions containing both blocked polyisocyanates and polyhydroxyl compounds.
2. Description of the Prior Art
U.S. Pat. Nos. 4,522,851, 4,098,933 and 4,284,544 are directed to the preparation of blocked polyisocyanates which are subsequently dispersed in water and used as crosslinkers for isocyanate-reactive compounds. The disadvantage of blocking the polyisocyanate before it is dispersed in water is that the blocked polyisocyanate is generally a solid or a high viscosity liquid which is very difficult or impossible to disperse in water without first diluting the blocked polyisocyanate in an organic solvent or heating the blocked polyisocyanate to an elevated temperature to reduce its viscosity.
Because one of the primary reasons for dispersing polyurethanes and polyisocyanates in water is to avoid the unnecessary use of organic solvents, it is certainly not desirable to have to dilute the blocked polyisocyanate with an organic solvent in order to reduce its viscosity sufficiently to disperse it in water. In addition, the extra energy costs and time which are necessary to heat the blocked polyisocyanate to an elevated temperature to reduce its viscosity are also undesirable.
It is possible to overcome these difficulties in accordance with copending application, Ser. No. 07/734,708, which discloses dispersing the polyisocyanate in water prior to reacting the polyisocyanate with a blocking agent which is more reactive with isocyanate groups than water, most preferably an oxime. The copending application also discloses the addition of isocyanate-reactive compounds for reaction with the isocyanate groups of the blocked polyisocyanate after release of the blocking agent, provided that they are added after the blocking reaction is complete. It is disclosed that these isocyanate-reactive compounds should be added after the blocking reaction. Otherwise, their presence could interfere with the reaction between the isocyanate groups and the blocking agent.
Therefore, one of the disadvantages of the process disclosed in the copending application is the need for an additional mixing step to blend the isocyanate-reactive compounds with the blocked polyisocyanate.
Accordingly, an object of the present invention is to is provide a process for preparing an aqueous dispersion of a blocked polyisocyanate in combination with a co-reactant which does not require an additional mixing step.
Surprisingly, this object may be achieved in accordance with the present invention as described hereinafter.